This invention relates to an optical color analysis process and more particularly to a video color recognition and comparison process for use in quality control, color grading, shade sorting, color durability testing and the like.
There are numerous commercial situations where an accurate evaluation and analysis of color is important. As an example, in the manufacture of pigmented plastics products, it is desirable that all products of the same type be approximately the same color. Generally this means that the color should be fairly close to a predetermined sample color. Exact color matching in all circumstances is not possible, so some color variation is generally accepted.
Other applications for the same type of process are in color grading, classification and sorting, shade sorting, and color durability testing (wherein color change from exposure conditions is calculated).
The most common method for color analysis is visual inspection by the human eye. The human eye has a capability of seeing a great number of colors and can discern fine differences in color shading. A problem with the human eye, however, is that it lacks memory, that is, the ability to recall exactly what a previously inspected color looks like. Thus, side by side comparisons are necessary in a manual color inspection system. Another problem with the human eye is that it lacks the capability of accurately determining how much different two dissimilar colors are and whether a dissimilar color fits within certain tolerance limits. The human eye also has difficulty in ascertaining the characteristics of color differences (i.e., whether one color contains too much red or too little blue or the like). Subjective and personal considerations also come into play in a purely manual viewing system.
Many attempts have been made to provide a more objective and measurable substitute for the human eye. One of the most prominent methods is the use of a spectrophotometer. In a spectrophotometer system, reflected light is broken into its component colors by means of a prism or a diffraction grating, and electro-optical sensors, such as photodiodes physically placed in the path of the various colors in the spectrum sense the energy levels for each color in the spectrum. The outputs of these photodiodes are reproduced in a graph showing the energy levels of the various wave lengths in the visible spectrum. These curves, called spectral distribution curves, can be compared with the spectral distribution curves of other objects in order to determine if the colors of two objects are the same.
Spectrophotometer equipment is quite expensive and has a number of drawbacks. The equipment sees only a single color averaged over the entire viewing screen and thus can analyze only one color at a time. Such equipment is also sensitive to differences in texture and ambient lighting conditions. Moreover the equipment does not produce output results that are always consistent with the human eye. Thus, the human eye may perceive a match, while the equipment indicates that the colors are different. Conversely, the human eye may perceive differences while the equipment indicates that the colors are the same.
It is an object of the present invention to provide a simpler and more effective system for color analysis and color comparison that provides reproducible results consistent with the observations of the human eye.